Optical coherence tomography or OCT (Huang, D. et al., 1991. Optical coherence tomography. Science 254: 1178-1181) is an interferometric technique, which allows obtaining the differences in optical path between surfaces. Using a lateral sweep scanner on a sample allows obtaining a collection of interferograms (A-scans), which form an image of the cross-section of the sample (B-scan). Scanning in both directions (x and y) allows obtaining a collection of B-scans and constructing a three-dimensional image of the sample. The axial resolution of the technique is in the order of microns (Povazay, B. et al. 2002. Submicrometer axial resolution optical coherence tomography. Opt. Lett. 27:1800) and is determined by the spectral bandwidth of the source (superluminescent diodes are typically used, although femtosecond lasers or swept-source lasers are also used). The interferograms may be obtained in the temporal domain, physically changing the length of the reference arm or, in the frequency domain, especially (spatial frequency domain (Fercher A. F. et al., “Measurement of Infraocular Distances by Backscattering Spectral Interferometry”. Optics Communications, 1995, 117:43-48)) or temporarily (temporal frequency domain (Chinn, S. R. et al., (1997). Optical coherence tomography using a frequency-tunable optical source. Opt. Lett., 22, 340-342)) codified by means of a spectrometer or scanning the frequency of the source.
The increased velocity with which data are acquired in the OCT systems (of up to 150,000 A-scans/s) has allowed the capture of three-dimensional images in less than 1 second. The high axial resolution (2-20 μm) and high lateral resolution (in the region of 100 μm) give optical coherence tomography high potential for topographical and profilometric characterization of surfaces and for the in vivo measurement of the corneal topography, amongst other things.
In the current state of the art (relative to optical coherence tomography techniques, ocular surface topography systems and profilometric techniques for general surfaces based on other methods), there is a need to quantify optical coherence topography systems, in order to improve the ocular biometry obtained by means of these systems and to therefore achieve a new, advantageous process associated with the use of profilometry based on optical coherence tomography. Furthermore, there is a need for a general method for calibrating optical coherence tomography systems, with the aim of improving the quantitative information obtained from these systems. In general, the use of the optical coherence tomography (OCT) technique as a topographical technique is limited by the presence of scanning distortion associated with the architecture of the sweep system (generally formed by a 2-axle mirror scanner), which also produces field distortion and astigmatism in the images. The main factor contributing to this distortion is the separation of the mirrors in the scanner and the focal length of the lens that collimates the beam on the sample and, to a lesser extent, the flatness of the mirrors and misalignment of the rotation beam of the mirrors.
Up until now, it remains unknown for the state of the art a general method for calibrating and correcting the scanning distortion which may be applied to any optical coherence tomography system without prior knowledge of the optical and mechanical configuration of the system. The absence of calibration and correction of the scanning distortion has prevented the quantitative use of the optical coherence tomography systems from becoming generalized, as well as the correct interpretation of topographical data. One of the main aims of the present invention is to provide a method for calibrating and correcting the scanning distortion in order to quantify the topographical data obtained using any Optical Coherence Tomography system. Correcting optical distortion is relatively simple in optical coherence tomography systems based on one single scanner and with two-dimensional acquisition of data. However, in systems with two scanners, with three dimensional acquisition of data, distortion is complex since it is not linear and has dependencies between the lateral and axial positions, as well as being dependent on the optical and geometrical configuration of each piece of equipment. This complexity has generally prevented quantitative three-dimensional topographical data from being obtained.
Various optical coherence tomography systems for the anterior segment of the eye exist on the market. These systems provide quantitative biometric data, generally in the axial direction. Nevertheless, the correction of the scanning distortion in these commercial systems has not been proven, as is the case in one of the most widespread previous commercial systems (Visante, Zeiss) based on Placido rings, despite it providing three-dimensional corneal elevation data. Some authors provide alternative scanning configurations that minimize scanning distortion depending on the mirror configuration of the same (Chin et al, (1997). Optical coherence tomography using a frequency-tunable optical source. Opt. Lett., 22, 340-342) or in scanner systems oriented towards cutting machines (Ireneusz Grulkowski et al, “Anterior segment imaging with Spectral OCT system using a high-speed CMOS camera”, OPT. Express 17, 4842-4858, (2009)). However, these systems always leave residual distortions, which should be corrected in order to be able to obtain the three-dimensional coordinates of each point of a surface.
Westphal et al., (Correction of geometric and refractive image distortions in optical coherence tomography applying Fermat's principle, Opt. Express 10, 397-404, (2002)) provides a solution to the scanning distortion in corneal OCT systems wherein the scanning system is a non-linear scanner system (with resonant mirrors with non-telecentric scanning), by means of axially taking images around the axial position, applying only to two-dimensional sections of the sample and not to three-dimensional ones.
Kim et al., (Automated analysis of OCT images of the crystalline lens, Proc. SPIE 7163, 716313, (2009)) use a telecentric system to acquire transversal (two-dimensional) images free of optical distortion. O'hara and Meyer (U.S. Pat. No. 7,878,651) propose the use of beams perpendicular to the cornea in order to obtain the refraction thereof. However, this does not produce the claimed distortion correction but rather it produces the opposite effect, as the beams have to transverse very different paths.
Ortiz et al. (Optical coherence tomography for quantitative surface topography, Appl. Opt. 48, 6708-6715, (2009)) proposed a method for optimizing the scanning distortion in a temporal domain OCT system as well as for three-dimensional correction of residual scanning distortion, based on acquiring axial images by means of a confocal lateral image channel built into the OCT system. However, this method requires the use of a confocal channel to obtain the scanning distortion. This confocal channel is not generally available in Optical Coherence Tomography instruments, which is why the method is not generally applicable. These authors furthermore provide theoretical estimations of the scanning distortions, which allow predicting the scanning distortion measured experimentally but these require precise knowledge of the optical and geometrical configuration of the instrument. The theoretical estimations allow obtaining an optimal configuration which in turn allow minimizing these distortions but not to eliminate them, it being necessary to carry out the proposed method for the residual distortions which remain in the optical illumination and light collection system.
The scanning distortion correction method, object of the present invention, may be applied to obtaining the profilometry of surfaces in general or specifically to corneal topography, by means of employing optical coherence tomography systems.
U.S. Pat. No. 7,416,300 describes the use of optical coherence tomography for metrology of lenses and surfaces but it does not mention the correction of the scanning distortion. U.S. Pat. No. 716,313 and U.S. Pat. No. 5,491,524 describe corneal topographic mapping systems by means of optical coherence tomography but they do not disclose the correction of the scanning distortion. Generally speaking, in these studies, the maps are obtained based on a set of cross-sections acquired by an assembly of meridians around a rotation shaft, centered in the corneal apex (in a similar way to Scheimpflug systems or crack rotation scanning systems), limiting the lateral resolution in the radial dimension.
Once the scanning distortion has been corrected, the optical coherence tomography technique is advantageous in comparison to surface contact profilometry (for example Talysurf), including faster data acquisition and the absence of contact with the sample. It is also more advantageous than optical profilometry based on microscopy, including greater operational distance, much faster data acquisition in wider areas and greater independence in terms of the specular reflection properties of the sample. Once the scanning distortion has been corrected, the optical coherence tomography technique is advantageous in terms of measuring corneal topography in patients, in comparison to corneal video keratoscopy based on Placido rings, usually employed in clinics, including greater axial and lateral resolution, in the radial dimension and the direct acquisition of elevation data, without suppositions derived from the presence of the skew ray. It is also more advantageous than corneal topography based on Scheimpflug, including faster acquisition and greater axial and lateral resolution.